Problem: Kevin is 4 times as old as Nadia and is also 12 years older than Nadia. How old is Kevin?
Explanation: We can use the given information to write down two equations that describe the ages of Kevin and Nadia. Let Kevin's current age be $k$ and Nadia's current age be $n$ $k = 4n$ $k = n + 12$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $k$ is to solve the second equation for $n$ and substitute that value into the first equation. Solving our second equation for $n$ , we get: $n = k - 12$ . Substituting this into our first equation, we get the equation: $k = 4$ $(k - 12)$ which combines the information about $k$ from both of our original equations. Simplifying the right side of this equation, we get: $k = 4k - 48$ Solving for $k$ , we get: $3 k = 48$ $k = 16$.